Following two days of warning strikes last week, the GdF union for air traffic workers said that a third round of stoppages, originally planned to run for 24 hours from 0400 GMT Monday, would now be extended to 48 hours due to the intransigence of airport operator Fraport.

Fraport, for its part, said that although 240 of a total 1,250 flights were cancelled Monday, it had been able to ensure around 1,000 flights, or more than 80 per cent.

"Despite the strike ... activity at Frankfurt airport is functioning at a good level of punctuality," the company said, adding that it was successfully ensuring intercontinental connections.

It said it expected similarly "good figures" for the 1,200 flights scheduled on Tuesday.

The wage dispute concerns 200 so-called "apron control" staff who direct aircraft in and out of their parking positions both in the control tower and on the tarmac.

The GdF union called for warning strikes after Fraport turned down mediation proposals in their wage dispute.

The strikes kicked off Thursday, grounding 172 flights, and intensified Friday with 280 flights cancelled, 250 of them by German flag-carrier Lufthansa, which uses Frankfurt as its primary hub.

A spokeswoman for Lufthansa said that 200 flights would be cancelled on Monday, followed by a further 160 on Tuesday.

According to documents made available by Fraport, the union is demanding pay rises of 25 to 50 per cent, depending on a worker's grade, as well as increased bonuses and reduced working hours.

Frankfurt airport is Europe's third busiest after London-Heathrow and Paris Charles de Gaulle.

GdF repeatedly threatened strike action last year in a long-running wage dispute for regular air traffic controllers.

A strike was averted when the union and Germany's air safety authority DFS reached a deal in court in October.